The two babies have different due dates. This is a rare development called superfetation - when a mother conceives another child while pregnant.

'It does really sound like this is a true case of different conception times for these children,' said Dr Karen Boyle of the Greater Baltimore Medical Center told ABC News in an interview.

'When the woman had her ultrasound initially, they saw one sack, one baby developing, and that baby had a certain gestational age; then they noticed a second heartbeat in a child that was much, much younger developmentally.'

Dr Boyle also said that she has only heard of 10 reported cases of superfetation.